The present invention relates, in general, to diaphragm pistons that operate in the cavity of a body in the manner of a piston and cylinder and, in particular, to such diaphragm piston arrangements in which the diaphragm exhibits an inherent "spring effect", which can be beneficial particularly when the diaphragm piston is employed to operate pneumatic valving and the like.
In railroad brake control applications, where it is common practice to employ high pneumatic pressures on the order of 100 psi., for example, a fabric-reinforced-type of diaphragm is necessary to withstand the high-pressure forces without diaphragm "balooning" and subsequent failure. These fabric-reinforced diaphragms tend to be stiffer than regular diaphragms and thus exhibit a substantially noticeable internal force or "spring effect". This so-called "spring effect" is an inherent force within the diaphragm itself that tends to exert a relatively light bias force on the diaphragm piston. This internal force is developed when the diaphragm is deflected from its molded-in configuration and typically acts in a direction to restore the diaphragm to its normal shape. In sensitive operating control valves, as in the well-known, industry standard, ABD type railroad brake control valve device, in which the service valve device 1 shown in FIGS. 1 and 2, is comprised of a diaphragm-type piston 2 that positions a slide valve (not shown) to achieve the desired brake control functions in response to variations in the brake pipe/auxiliary reservoir pressure relationship acting across the diaphragm piston, it is desirable to actuate the piston at very low pressure differentials in order to position the slide valve and achieve the resultant control function without delay. This is particularly desirable in actuating the piston from its release position, as shown in FIG. 2, to its application position, as shown in FIG. 1, and relies upon the diaphragm "spring effect" to help achieve this purpose.
Because of the relatively long service life required of diaphragms used in the above-mentioned application, conical-type diaphragms, as disclosed in U.S. Pat. No. 3,173,342 and incorporated herein by reference, are typically employed. The significantly long service life attributed to these conical-type diaphragms is achieved by maintaining the fabric material uniformly embedded in the rubber that comprises the diaphragm proper. This is possible since the normally flat fabric material is not required to assume an unnatural or convoluted shape during the molding process, as in bellows-type diaphragms, for example, and therefore does not tend to shift toward the surface of the rubber during the vulcanizing process. In realizing a long service life, however, due to the fabric material in conical-type diaphragms being unstressed during the vulcanizing process, these conical-type diaphragms also exhibit a relatively light "spring effect" for the same reason.
Consequently, the efficiency of the control valve device employing such conical-type diaphragms is compromised with respect to achieving fast brake response. Moreover, the convolution in these conical-type diaphragms has been found to take an inside-out set over a period of time, which further reduces the diaphragm "spring effect" and contributes to the decline in brake response.
It will be understood, for example, that during a brake release, a relatively high pressure differential is created across piston 2, thereby causing convolution 3 of diaphragm 4 to become inverted from its normal upwardly-directed disposition during movement of piston 3 from application position (FIG. 1) to release position (FIG. 2). Once movement of piston 2 to brake release position is complete and the pressures across piston 2 become substantially equalized, the inherent "spring effect" of diaphragm 4 is intended to gradually force the diaphragm convolution 3 to automatically unfold or revert back to its normal upward disposition, as shown in FIG. 2. Piston 2 is, therefore, in readiness for immediate actuation to application position in terms of the "spring effect" being in the desired direction to encourage movement of piston 2 toward application position. Also the volumetric displacement between the pressure chambers 6 and 7 on opposite sides of the diaphragm piston, due to transition of the diaphragm convolution, will have occurred prior to a subsequent reduction of brake pipe pressure to initiate a brake application.
In the event, however, the diaphragm convolution 3 does no revert back to its normal position following release of a brake application; and, since the piston normally remains in release position for a considerably long period of time between brake applications, the diaphragm convolution 3 tends to take a set in an inverse or downward disposition, as shown in FIG. 2. The result of this is that the initial upward-acting "spring effect" is lost and the set resists further diaphragm movement. Consequentially, a higher than normal pressure differential is required to actuate piston 2 when a brake application is subsequently initiated, thus increasing response time and adversely extending the time required to obtain braking.
Moreover, this requirement to obtain a higher than normal pressure differential to actuate piston 2 is aggravated by the fact that displacement of the diaphragm convolution also results in a volumetric exchange between chambers and 7 that tends to counteract development of the required pressure differential necessary to actuate piston 3, thereby further delaying piston response.